1. Field of the Invention
The present invention relates to position detectors for use in inputting graphics and text to computers.
2. Description of the Related Art
One type of known position detector is disclosed in Japanese Unexamined Patent Application Publication No. Kokai Sho 63-70326(U.S. Pat. No. 4,878,553).
In this position detector, electromagnetic waves are transmitted between a tablet and a position indicator, whereby coordinate values of a position indicated by the position indicator are detected. Of a plurality of loop coils which are provided in the tablet and which are parallel to position detecting directions, the loop coils are sequentially selected, and the selected loop coil emits electromagnetic waves. The electromagnetic waves which are reemitted from a resonant circuit provided in the position indicator are received, and coordinate values of the position indicated are thereby detected on the basis of a distribution of reception signal strengths.
This type of position detector is often provided as an input device for a computer and is often used with a CRT monitor which is a display. Since the CRT monitor controls electron beams by a strong magnetic field, the CRT monitor emits strong electromagnetic noise. The above-described position indicator is influenced by noise emitted from the CRT monitor, and coordinate values cannot be reliably detected.
In addition to noise from the CRT monitor, this type of position indicator may be influenced by electromagnetic waves from radio stations.
Efforts have been made to suppress the influence of external noise. In the above-described example, transmission/reception is performed multiple times for each loop coil, and averaging is performed to reduce the influence of noise. A band pass filter which preferentially receives a desired frequency component is provided as a receiving circuit.
When a frequency component included in noise from the CRT monitor and electromagnetic waves from the radio stations matches the frequency to be detected by the tablet, the foregoing methods cannot eliminate the influence of noise from the CRT monitor and electromagnetic waves from the radio stations.
Accordingly, it is an object of the present invention to provide a position detector for reliably detecting coordinate values without being influenced by noise even when a frequency component of external noise matches the frequency to be received.
In order to achieve the foregoing objects, a position detector is provided including a position indicator including a resonant circuit which includes at least a coil and a capacitor; a plurality of loop coils which are parallel to position detecting directions; and a tablet for detecting coordinate values of a position indicated by the position indicator on the basis of the levels of reception signals generated at the loop coils when electromagnetic waves at a predetermined frequency are transmitted/received relative to the position indicator. The tablet includes a transmission/reception control unit for consecutively performing transmission/reception of electromagnetic waves relative to the position indicator at least twice while the same loop coil is being selected; a phase difference control unit for controlling a phase difference of the signals in each of transmission/reception when transmission/reception of electromagnetic waves is consecutively performed at least twice; a synchronous detector circuit for detecting synchronization by using a signal obtained by controlling the phase of the reception signal generated at the loop coil as a detection clock; and a signal level processing unit for computing the average or the sum of the levels of at least two reception signals which are detected by the synchronous detection and which are based on transmission/reception of electromagnetic waves which is performed at least twice.
Arranged as described above, when transmission/reception is consecutively performed at least twice, the position detector can adjust a phase difference of signals exchanged between the position indicator and the tablet each time. When computing the average or the sum of signal levels detected by these transmission/receptions, the phase difference can be adjusted so that influence of noise can be cancelled out. Therefore, coordinates can be reliably detected without being influenced by external noise.
The phase difference control unit can be implemented by changing start timing for transmission/reception of electromagnetic waves each time, and hence the structure can be simplified.
According to the present invention, transmission/reception of electromagnetic waves relative to a position indicator is consecutively performed at least twice, and the time between the first transmission/reception and the second transmission/reception is adjusted. Thus, the polarity of noise detected in the first transmission/reception and second transmission/reception is inverted, thereby canceling out the noise. Accordingly, a position detector can be provided which is capable of reliably detecting coordinate values of a position indicated while suppressing influence of external noise which contains many frequency components at the same frequency as the resonant frequency of the position indicator, which are difficult to remove by using a known position detector.
One aspect of the invention comprises a method of detecting a position of a resonant circuit with respect to a sensor. The sensor has a plurality of loop coils for detecting coordinate values of a position indicated by a resonant circuit as a function of reception signals generated at the loop coils when electromagnetic waves are transmitted to and received from the resonant circuit. A first signal is sent from the sensor to the resonant circuit that has a frequency and a first phase angle, and a second signal is received from the resonant circuit in response to the first signal. Then a third signal is sent from the tablet to the resonant circuit that has the same frequency as the first signal but a second phase angle. A fourth signal is received from the resonant circuit in response to the third signal, and the second signal and the fourth signal are averaged to cancel noise having frequency components close to the frequency.
Another aspect of the invention comprises a method of detecting a position of a resonant circuit with respect to a sensor having a plurality of loop coils for detecting coordinate values of a position indicated by a resonant circuit as a function of reception signals generated at the plurality of loop coils when electromagnetic waves are transmitted to and received from the resonant circuit. The approximate position of the resonant circuit is detected by sending a preliminary signal to the resonant circuit from each of the plurality of loop coils, and then the strength of the signal returned by the resonant circuit in response to each of the preliminary signals is detected. A subset of the plurality of loop coils at which the signal received from the resonant circuit in response to the preliminary signal has a strength exceeding a predetermined level is selected. A first signal having a frequency and a first phase angle is sent from a selected one of the subset of the plurality of loop coils to the resonant circuit, and a second signal is received from the resonant circuit in response to the first signal. A third signal having the frequency and a second phase angle is sent from the selected one of the subset of the plurality of loop coils to the resonant circuit, and a fourth signal is received from the resonant circuit in response to the third signal. The second signal and the fourth signal are averaged to cancel noise having frequency components close to the frequency. These steps are repeated for the remaining loop coils in the subset, and the coil for which the average of the second and fourth signals satisfies a given criterion is selected.
A further aspect of the invention comprises a position detector including a position indicator having a resonant circuit and a sensor comprising a plurality of loop coils for detecting coordinate values of a position indicated by the position indicator as a function of reception signals generated at the loop coils when electromagnetic waves at a predetermined frequency are transmitted to and received from the position indicator. The sensor comprises a transmitter for sending signals having the predetermined frequency and a phase angle to the resonant circuit and a receiver for receiving signals from the resonant circuit. The sensor also includes a controller for controlling the phase angle and a processor operably associated with the receiver for computing the average of at least two out-of-phase signals transmitted by the transmitter so that noise having frequency components similar to the predetermined frequency is substantially canceled.